The invention relates to an antenna arrangement for personal radio tranceivers, in which the transceiver is connected to a resonant antenna which is shorter than a quarterwavelength of radio signals to be sent and received.
The term "personal radio transceiver" designates a portable radio transmitter and receiver set which has a battery supply, its operational frequency falls in the VHF or UHF band and the maximum high frequency output power is below 5 W. In operation the set is held in hand closely to the human body and the antenna of the set is connected directly to the housing of the transceiver.
The design of personal transceivers is always a compromise between several mutually conflicting requirements. In view of its handling it is preferable if the set has small dimensions and weight, however, with small weight and size the output power and the maximum operating time is decreased. The operating time is determined by the output power and the useful life of the battery supply. The size and design of the antenna can significantly determine the performance of such transceivers. In personal radio transceivers the effective radiation of the available high frequency power is rather problematic due to the vicinity of the human body, therefore the design of the antenna is a decisive factor regarding the operational properties of the transceiver.
If the properties of personal radio transceivers are compared to the radiational properties of a quarterwave vertical whip antenna which is arranged on a sufficiently large metal surface, it will be observed that, with identical output power, the established electromagnetic field of such transceivers will be about 10 dB smaller than for the whip antenna.
In the paper by N. H. Sheperd and W. G. Chaney entitled "Personal Radio Antennas" /IRE Trans. Vehicular Comm. Vol. VC-10 pp. 23-31, April 1961/ the results of measurements carried out by various types of "small" antennas are summarized. Here the conclusion has been drawn that the quarterwave whip antenna is the most favourable and it has an attenuation of about 10 dB compared to the ideal antenna with 0 dB gain. The various other types of shortened antennas were by 3 to 10 dB worse than this quarterwave whip.
In addition to the problem of attenuation there is a further problem with such "short" antennas i.e. the fluctuation of the field strength during operation caused by the varying relative position of the set and of the human body. The extent of such fluctuation can be about 5 dB.
The small effectivity of radiation which is below 10% can be explained by the fact that the housing of the transceiver has a size which is negligably small compared to the wavelength, thus it can not act as a counterweight for the radiating antenna. From this it follows that a portion of the antenna current will flow through the hand which supports the set, into the human body which has a small conductivity, and the corresponding power is dissipated. The presence of the human body increases the base point impedance and decreases the current of the antenna.
When the human body is close to the voltage maximum of the radiating antenna, then the established electrical coupling might de-tune the antenna, can also change its impedance and in addition to the radiation losses caused by the presence of the body, mismatching losses will occur. This latter effect is particularly significant in the so called miniature antennas built of a helical radiator of normal mode of radiation, because such antennas get very close to the human body during operation and the detuning effect of the body can therefore be excessive. This is a rather serious problem because the reactance steepness of the base point impedance of such shortened antennas are rather high and when detuning takes place, the mismatching losses will be substantial.
In addition to the above sketched problems a further problem lies in the shielding effect of the human body which can only be decreased by raising the height of the antenna. This latter is conflicting, however, with the demand of miniaturization and of comfortable handling.